Sub-nanoscale electronic systems and devices

ABSTRACT

A new class of electronic systems, wherein microelectronic semiconductor integrated circuit devices are integrated on a common substrate with molecular electronic devices.

This is a continuation application of Ser. No. 07/893,092, filed Jun. 1,1992 now U.S. Pat. No. 5,475,341.

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All of the material in this patent application is subject to copyrightprotection under the copyright laws of the United States and of othercountries. As of the first effective filing date of the presentapplication, this material is protected as unpublished material.

Portions of the material in the specification and drawings of thispatent application are also subject to protection under the maskworkregistration laws of the United States and of other countries.

However, permission to copy this material is hereby granted to theextent that the owner of the copyright and maskwork rights has noobjection to the facsimile reproduction by anyone of the patent documentor patent disclosure, as it appears in the United States Patent andTrademark Office patent file or records, but otherwise reserves allcopyright and maskwork rights whatsoever.

BACKGROUND AND SUMMARY OF THE INVENTIONS

The present inventions relate to a new generation of electronicmicrocircuit technology, having dimensions much smaller than those ofsemiconductor integrated circuits, and to related systems and processes.

To better explain the significance and advantages of these innovations,the following paragraphs (down to page 15) will review sometechnological context. This technological context is not necessarilyprior art, but is intended to help in pointing out the disclosedinventions.

The Era of Downscaling

Since about 1960, the steady downscaling of integrated circuit minimumdimensions has permitted ever-increasing density, and thus anever-increasing range of functionality at an ever-more favorable cost.This wealth of opportunity has permitted system designers to introducemany of the electronic products which have revolutionized industry anddaily life in these decades. Continued downscaling steadily improves theavailable functionalities and pricing, and thus steadily challengessystem designers. This fosters a continuing elimate of active innovationand competition.

The most obvious index of downscaling is the steady reduction in the"minimum geometry" which can be specified for fabrication of anintegrated circuit. This corresponds to a reduction in the size andspacing of the individual transistors, and thus steadily increases thenumber of transistors which can be fabricated in a given area. However,it is important to note that scaling has also provided exponentialimprovements in device speed and power dissipation, which has led tosubstantial enhancement of system performance. Thus, an end to the epochof downscaling would drastically reduce the speed of progress inelectronics.

Limitations of Semiconductor Microelectronics

The danger now in sight is that the downscaling of minimum geometries oftransistor-based integrated circuits will eventually be brought to anend by a combination of problems related to devices, interconnections,noise, and reliability.¹ The resulting saturation of circuit densitiesalmost certainly implies a saturation of the historical exponentiallydownward trend in cost and volume per bit or function.

Several constraints are visibly converging to cut off the advantages offurther scaling. While it is likely that clever process modificationscan postpone the impact of some constraints, it does not seem likelythat all can be avoided.

A technology-dependent issue is where existing ULSI ("ultra-large-scaleintegration," i.e. semiconductor fabrication with minimum dimensions ofa micron or less) will usefully end. From recent work, it is reasonablethat this will occur in the 0.1 micron regime; scaling to just the 100sof Å, level may not be cost-effective in relation to the developmentcosts of the technology. Thus, identifying an atomic-scale devicetechnology seems the only approach worth the investment.

Alignment Tolerances

One of the basic problems is alignment tolerances: formation of featuresat a small minimum size λ does not imply that that minimum size can beused for fabrication of working circuits: it is also necessary to havealignment tolerances which are much smaller than λ (preferably wellunder λ/4). (Thus, a 0.8μ lithography process will typically have aspecified alignment tolerance of ±0.15μ or less.)

With further scaling, this imposes several nonobvious difficulties. Oneis thermal stability, as discussed below.

Dopant Diffusion Lengths

Diffusion lengths scale approximately as (Dt)^(1/2), where t is time atthe highest temperature, and D is a temperature-dependent diffusionconstant characteristic of the dopant and the background material. Asdimensions are reduced, the dopant diffusion lengths in silicon areposing difficulties in process design. In the past decade, manyaccommodations have been made to reduce dopant mobility and to reducetime at high temperatures. However, it is not clear that suchaccommodations can be continued indefinitely. For example, arsenic (orantimony) dopants are now used increasingly in place of phosphorus, butthere is no suitale N-type dopant with significantly lower diffusivitythan these two.

Punch-through, Doping Levels Electric Fields, and Hot Electrons

A voltage applied across a semiconductor junction (in the reverse-biasdirection) will naturally create a depletion region around the junction.The width of the depletion region depends on the doping levels of thesemiconductor. If the depletion region spreads to contact anotherdepletion region, "punch-through," i.e. uncontrolled current flow, mayoccur.

Higher doping levels will help to minimize the separations required toprevent punch-through. However, if the voltage change per unit distanceis large, this creates a further difficulty.

A large voltage change per unit distance implies that the magnitude ofthe electric field is large. An electron traversing such a sharpgradient may be accelerated to an energy level significantly higher thanthe minimum conduction band energy. Such an electron is known as a "hot"electron, and may be sufficiently energetic to pass through aninsulator. Thus, hot electrons can irreversibly degrade some commontypes of devices.

Isolation in a Monolithic Semiconductor Substrate

Conventional semiconductor integrated circuit technology uses amonolithic substrate which is all one crystal. Such substrates providegreat advantages in processing. However, this device architecture posessome inherent difficulty with further scaling. One difficulty is lateralisolation of devices from each other. Another difficulty is leakagecurrent scaling. Another difficulty is presented by the diffusivity ofcarriers within the substrate: free carriers (generated, e.g., by analpha particle hit) can diffuse over many tens of microns to helpneutralize a stored charge. Some attempts have been made to overcomethese difficulties by using total isolation from the substrate, but todate such technologies have not demonstrated favorable economics ofscaling.

Considerations in Further Downscaling

Theoretically, further downscaling of devices would still be achievablewith the appropriate device technology, IF the approach couldsimultaneously address the interconnection, reliability, and impliedfabrication limitations. Estimates based on abstract physical switchingdevice models which are independent of specific device technologiesindicate that several orders of magnitude in downscaling of device powerin devices would be theoretically permitted,² if an appropriate devicetechnology could be found. The key to this search is to employelectronic phenomena which are characterized by dimensions much smallerthan the depletion layer widths and diffusion lengths which provide thebasis for conventional transistor function.

Limitations of Semiconductor Nanoelectronics

Within the last decade, tremendous progress in semiconductornanofabrication and nanoscale spatial and charge quantization phenomenahas bridged the gap from the 0.1 micron regime to the ˜10s of Å scale,and even to the atomic level with scanning probe techniques.³ Theseadvances allow one to create electronic structures that exhibit manifestquantum and single electron effects. However, proposed solid statedevice implementations at this level suffer from three problems. Thefirst is critical dimensional control. Electron devices which operate inthis range must operate by tunneling, since a barrier (heterostructure,oxide, or otherwise) is a prerequisite for isolation in a 3-terminaldevice that can exhibit gain. However, electron tunneling isexponentially sensitive to atomic-layer fluctuations in the tunnelingbarriers, resulting in device characteristic variations unacceptable forlarge scale integration. Secondly, device embodiments utilizing discreteelectron charging (single electron transistors, or SETs) suffer fromreduced operating temperatures; for room temperature operation, 1 nm orless size junctions are required, dimensions which imply severe tunnelbarrier fluctuation problems for solid state embodiments. Finally, noneof these approaches address the interconnection and alignment problems.It is instructive to note that these investigations have had littleimpact on extending ULSI, due to the fundamental limitations ofconventional semiconductor devices and fabrication. Fabrication at thenanoscale, and ultimately at the atomic level, of even the simpleststructures (interconnects and contacts) is a daunting task. Techniquessuch as electron beam and STM⁴ lithography for pattern transfer appearsto bottom out at ˜100 Å, due to the requirement of pattern transfer.Atomic manipulation with scanning probes has been demonstrated, but itis unlikely that this technique will be more than a laboratory curiositysince it is essentially a serial approach. (This criticism also holdsfor STM micromachined arrays, due to registration and access timelimitations).

Solid state embodiments of quantum size electronic devices suffer from anumber of problems. They are:

Dimensional Fabrication Tolerance: In a quantum device that utilizesquantum size effects, the intrinsic energy levels (and therefore thethreshold voltage) are at least inversely proportional to the size ofthe device, dependent on the detailed shape of the device potential. Ifthere are fabrication-induced dimensional variations, the quantum stateenergy will be different from device to device. The smaller the devicebecomes, the larger the voltage fluctuations. For many semiconductors,such as silicon and gallium arsenide, it is impossible to both make thedevice small enough such that the quantum energy level spacing is largecompared to room temperature thermal energy, and large enough such thata fluctuation of a single monolayer does not unacceptably shift thethreshold voltage.

Fabrication Tolerance Limits: Fabrication tolerance is critical when atunnel barrier (semiconductor or metal oxide) is used. The currenttransmitted through the tunnel barrier is exponentially proportional tothe tunnel barrier thickness, so again one has the limitation of changesof a single monolayer from device to device in a ULSI circuit willdrastically change the output current, and therefore the input voltageto the next stage. The only way to reduce this intrinsic problem (otherthan a fabrication scheme which guarantees atomic precision) is toincrease the barrier thickness to the point where a monolayer thicknessfluctuation does not affect the overall current. However, thisdrastically reduces current density, and thus does not make a gooddevice. Most useful semiconductor and metal oxide tunnel barriers are inthe range 5-10 monolayers.

Contact Statistics: When one makes a quantum device, the contacts to thedevice must also be reduced to this dimension. If the ohmic contactbetween devices is made too small, the wavefunction of one device willoverlap the second device. This has been demonstrated in high mobilitytwo-dimensional gas layers, where the change of the wavefunction in onepart of the layer remotely affected another part. This is notacceptable, since electron devices as we know them must have isolationfrom one to the next. This implies that the minimum distance betweendevices is the inelastic scattering length, which is approximately a fewtens of nanometers in useful semiconductor materials. Since this definesa minimum contact volume (i.e., a few hundred cubic Angstroms), we canestimate the number of dopant atoms in the contact, which for this sizeis only a few tens of dopant atoms. This means that the statisticalfluctuations in the number (and position) will dramatically shift thevoltage threshold.

Temperature and Voltage Limits: Quantum wave mechanical devices suffernot only from the above mentioned fabrication fluctuation problems, butalso from low temperature/voltage intrinsic limitations. A wavemechanical interference device may be conceived where the output ismodulated by an external gate or potential. However, destructiveinterference of the waves implies that the wave is monochromatic; thisimplies that only one subband can be used. Not only does this imply verylow temperature operation (the electron energy distribution at the Fermilevel must be much less than the room temperature thermal distribution),but the maximum conductance of the device is intrinsically very low (80μS).

Proposed "Waveguide" Devices: A different proposed structure is the"waveguide" type of device, in which it has been suggested that theelectron wavefunctions will remain in a standing wave pattern which canbe changed by inducing a reactance shift at a control point (analogousto an RF stub tuner). However, this proposal has a difficulty due to themultiple subbands available for electrons in semiconductors: sincce thedifferent subbands will typically have different effective wavelengthsin a physical structure, the phase shifts which switch the lowestsubband off will not necessarily switch off the higher subbands.

The Interconnect Problem: Even if a technology can be identified whichsolves the device scaling problem, the problems of interconnections andreliability will require revolutionary solutions. The "interconnectproblem" is manifested as propagation delays in device interconnectionsthat limit ultimate system performance. This is especially crucial forthe complex logic circuitry of general-purpose processors. Thoughincremental evolutionary improvements in present transistor technologywill surely continue, the resultant yield loss and subsequent increasein cost-per-function will eventually reach a minimum. An interestingexample of these limitations is the problem of achieving large dynamicrange alignment in this regime. Imagine that in the future one couldachieve 100 Å pitch and size active devices, which corresponds toapproximately 1 part in 10⁷ dimensional resolution when approximatelyconserving present chip dimension, for cost and function scaling. Thisimplies optimistically demanding less than 0.05° C. temperaturegradients during any fabrication step requiring alignment, which areclearly untenable dimensional and thermal requirements.

The ultimate device technology (if it exists) at this scale, independentof device embodiment, will thus solve the interconnection problem andwill be predominantly self-aligned.

The generic properties of a technology which addresses the criticalproblems can be detailed as follows:

a key innovation must be the solution to the interconnect problem.

The fabrication technology must be predominantly self-aligned,

perhaps non-lithographic and self-limiting.

Scaling to the atomic level, and room temperature operation, is desired.

Conjugated Conductive Pollers⁵

A vast amount of work has been done, by chemists and physicists, instudying the structure, synthesis, and electronic behavior of conjugatedconductive polymers.⁶ For many years these materials were not candidatesfor commercial applications; but more recently newer families ofmaterials have been identified.

π-orbitals and Extended States

"Conjugated" conductive polymers are those which have overlappingπ-orbitals. The resulting extended molecular orbitals provide a pathwaythrough which electrons can travel, IF an external field is applied andIF excess electrons are present, to provide conduction.

Note that conjugated bonding is not itself sufficient to provide goodconduction. Therefore, conductive polymer molecular structures ofteninclude "dopant" atoms which are selected to provide an adequate carrierdensity for conduction.

Improvements in Conductivity

Modern conductive polymer compounds have achieved bulk conductivities ofgreater than 1 Scm⁻¹. This begins to be comparable with metals. (Forexample, the bulk conductivity of copper is slightly less than 600Scm⁻¹.)

Improvements in Stability

Dramatic improvements have occurred in chemical stability of conductivepolymers. The first extensively studied material was polyacetylene,which is unstable and highly reactive with oxygen, but a succession ofinvestigators have found more stable and less reactive materials withhigher conductivities, as detailed below.

Innovative Systems, Modules, Circuits, Devices, and Methods

The present application discloses a novel technological approach whichfits these requirements, and can lead to a new era in ultra-denseelectronic systems.

Among the disclosed innovations is self-aligned spontaneous assembly ofchemically synthesized interconnects, active devices, and circuits. Thisis a revolutionary approach for spontaneously assembling atomic scaleelectronics. It attacks the interconnection and critical dimensioncontrol problems in one step, and is implicitly atomic scale.Concurrently, the approach utilizes an inherently self-aligned batchprocessing technique which addresses the ultimate fabricationlimitations of conventional ULSI.

There has been sporadic discusssion of molecular electronic devices forsome years now. However, one key deficit of all previous proposals istheir failure to solve the problem of achieving electrical GAIN in amolecular electronic device. The technology disclosed below provides atrue gain modulation, by modulating the electron wavefunction of apolymeric conductor.

The innovative technology disclosed herein also radically improves theeconomics of downsizing electronic devices. In conventionalsemiconductor technology, the cost per transistor is no longerdecreasing with reduced size; but the disclosed innovative technologyreturns to a regime of more favorable cost evolution.

The innovative technology disclosed herein provides an inherently veryhigh degree of self-alignment in processing. Moreover, this newtechnology is inherently very well suited to batch processing. Many ofthe problems of fabrication tolerance, which limit the further progressof conventional methods, are solved in the new technology by chemicalpurification and selection techniques.

New Interconnect Technology

Among the many innovations disclosed herein is a new self-alignedintegrated circuit interconnect technology which uses conductivepolymers. This technology has many features in common with the activedevice embodiments described below, but can be exploited independentlyof those embodiments.

Self-Assembling Wires

There exist non-semiconductor candidates for atomic scale electronicstructures which are presently at the molecular level. Since the 1970s,researchers have been exploring 1D conductive organic polymers, such aspolyacetylene. Advances in synthesis have identified more promisingcandidates, such as diphenylpolyene, polythyolenes, polyarylenevinylene,polyarylene, polyphenylene, and polythiophenes. Conductivities of thesewires (such as doped polyacetylene) have approached that of copper.⁷These organic chains can have long electron delocalization lengths; forexample, delocalization lengths of 20-34 atoms can be calculated fromdiphenylpolyene results,⁸ and ˜50 Å for polythiophenes.⁹

Though the synthesis of 1D molecular wires has been known for some time,the inability to manipulate and assemble organic structures into usefulcomplexes in a manner analogous to semiconductor devices has hinderedany application toward electronics. The isolation and measurement of asingle organic 1D wire, a key step toward electronic utilization ofconductive polymers, has yet to be demonstrated (though the conductivityof large assemblages of the material has been measured). Yet theutilization of the atomic-scale control inherent in organic synthesiscould provide an elegant solution to the fundamental fabricationlimitations described previously.

The present application presents a new approach which combines molecularsynthesis and nanofabrication. We take a conductive polymer, and attach("functionalize") onto the ends a compound that can selectively attachto a metal probe. Numerous examples of these "self-assembling"compound/metal pairs are known; for example, n-alkanethiols onto Au,isonitrile onto Pt, and alkanecarboxylic acid onto aluminum oxide.¹⁰This is in essence a conducting string with sticky ends, which couldbridge a gap between metallic contacts (of the selective metal). Byfabricating (by E-beam or STM) closely-spaced metallic contacts, themolecular wire can be spontaneously deposited from solution. Note thatif the molecular wire is synthesized with different end groups onopposing ends, the polarity of the attachment can be defined. Thespecific contact resistance of such an ohmic contact is not yetprecisely known, though the large value of the bond energies imply thismay not be a problem; for the organic thiolates and Au, this is 40-45kcal/mole. These "selective-attachment conducting polymers"(specifically, conjugated organic oligomers with functionalizedselective attachment termini) provide a technique for spontaneouslygenerating contacts between metallic endpoints, at the molecular scale(10-100 Å).

An advantageous application is for simple self-aligned interconnects;given a device with metal A on one terminal (for example, collector),and a second device with metal B on one terminal (for example, base), amolecular wire with end groups A' and B' (which attach selectively to Aand B, respectively) can bind selectively to make an interconnect,without a lithography step. Though we will see that interconnects arenot the most important application, this spontaneous "lock-and-key"concept is the basic ingredient. Also note that this process is, to adegree, length dependent. Interconnections of contacts separated bylonger than the designed molecular wire length are prevented. Animportant technology issue is the nuisance of unwanted binding of thepolymers other than at the terminal ends. It appears that this concerncan be solved for large metallic contacts (other than simple bindingposts) by either selective exposure of the metal (i.e., in the simplestcase by via holes) through an insulating overlayer coating at only thecontact points desired, or by post-attachment scavenging of the unwanteddangling molecular wires.

Selective Auto-connection to Terminals

The disclosed process innovations provide a self-aligned connection ofmolecular "wires" to their target terminals. If the deposition processis allowed to go to completion, the number of polymer chains connectedin parallel will be determined by the available area of thesemiconductor or metal contact which the chains are attaching to.

Active Device Operation

One class of sample embodiments operates using the principle of resonanttunnelling.

FIG. 1A shows a resonant tunnelling device in the on-state. Note that anenergy level in the well region provides an allowed transition forelectrons which tunnel through the barrier into the well. Such electronscan then tunnel through the second barrier to a region of lowerpotential, providing a net current.

FIG. 1B shows the device of FIG. 1A in the off-state (after thepotential of the base has been shifted). In this state the well nolonger has an allowable energy state at the potential of incomingelectrons. Therefore, normal conduction electrons cannot tunnel throughthe two barriers sequentially.

These Figures provide a simple schematic representation of a principleof operation which has been extensively analyzed, and which has beenrealized in heterojunction semiconductor devices. In such devices, thewell region must be physically very small to produce the neededseparation of allowable energy states, and these small dimensions causethe fabrication difficulties reviewed above.

However, the innovations disclosed in the present application provide adifferent way to achieve the same principle of operation (and also otherprinciples of operation). Polymeric molecular structures are manipulatedto produce combinations of well and barrier regions, with connections sothat the well and/or barrier potentials can be manipulated.

FIG. 4A shows the spatial variation of conduction band (CB) and valenceband (VB) energy levels across a first example monomer unit which canform conjugated conductive polymer structures. FIG. 4B shows the spatialvariation of conduction band (CB) and valence band (VB) energy levelsacross a second example monomer unit which can form conjugatedconductive polymer structures. FIG. 4C shows how, when two such monomerunits are chemically combined, the resulting dimer structure has a bandstructure which produces a barrier-well-barrier-well-barrier profile.

FIGS. 5A and 5B are a corresponding pair of drawings of two states ofoperation of a novel molecular electronic device. FIG. 5A shows the ONstate. In this state an energy level in the

well region is aligned with the energy level of incoming electrons, andthus resonant tunnelling can occur, to

produce a net flow of electrons from the "emitter" terminal through tothe "collector" terminal.

FIG. 5B shows the OFF state. In this state a different potential hasbeen induced at the "base" terminal. This induced potential propagates,through the chain X, to change to energy levels in the well region. As aresult of this change, no energy level in the well region is alignedwith the energy level of incoming electrons, and thus resonanttunnelling does not occur, and therefore current flow does not occurbetween the "emitter" terminal and the "collector" terminal.

Modulation of Conductor's Conductivity

With a conductive polymer (unlike a semiconductor structure) there aretwo ways to change the conductivity of the structure. FIGS. 5A and 5Bshow one architecture, in which the well potential is modulated toachieve gated resonant tunnelling. However, another alternative is tomodulate the BARRIER height, as shown in FIGS. 2A and 2B. In thisalternative, the modulator chain would be coupled to a barrier locationrather than to a well location.

The "Base Isolation" Barrier

To connect the modulator chain to the conductor chain, a coupling unitis preferably used which corresponds to a well in the prinary conductorchain. From the base connection point, the modulator chain is (in thepresently preferred embodiment) highly conjugated for short period; thena relatively high barrier is interposed, then a well, then a lowerbarrier; then the modulator chain is conductive for as long as needed.The barrier_(high) -well-barrier_(low) structure serves, in effect, as abase isolation barrier. Note that gain would not be possible withoutsome form of "base isolation." Thus, this feature of the architecturegives substantial advantages.

Electrical Asymmetry of the Active Device

To get electrical asymmetry between Emitter→Collector andCollector→Emitter operation, different barrier heights can be used ondifferent sides of the modulated tunnelling region. Moreover, theposition of the modulated tunnelling region within the conductiveoligomer chain can easily be made asymmetric if desired.

Connecting Signals into the Coupling Chain

Several methods are disclosed for coupling an input signal into themodulator side-chain of an oligomeric active device. The simplestconnects the side-chain to an electrical contact. Another disclosedmethod uses a photosensitive compound to generate a voltage shift underillumination. Another disclosed method uses direct coupling of themodulator side-chain (the "base") of one active device to the outputchain (the "collector") of another.

Also disclosed is a self-aligned contact process for preparing metalpads for the oligomeric conductors to bond to.

Inorganic Starting Structure

Preferably a semiconductor integrated circuit structure provides thestarting point for fabrication of moecular devices. The conventionalstructure provides a transition from macroscopic signals down to thesmall magnitudes characteristic of molecular electronics. In particular,conventional integrated circuit structures can advantageously provideinput ESD protection and output drivers.

Isolation

The isolation problem is not nearly as severe as in semiconductordevices, since there is no continuous substrate for carriers to diffusethrough. Conduction normally occurs along a single molecule, and theconnections of those molecules are largely defined by the formationprocess.

Device Density

Note that the technologies disclosed herein are inherently suitable for3D fabrication--as opposed to any planar technology, in which morelayers implies more process steps.

Interconnect Density

The novel interconnect technologies disclosed herein provideself-aligned interconnects which are length-constrained, but are NOTlimited to line of sight. For example, a molecular electronic activedevice could even be positioned in an undercut trench if desired.

Configuring SSI-equivalent Gates

It is also easy to configure devices with multiple inputs. For example,the detailed structure of a NOR gate is described below.

Passivation

Not all conductive polymers are as reactive as polyacetylene, but allare at least somewhat prone to react with O₂. On general, doped polymersare more reactive toward owygen than are the corresponding undopedpolymers.) However, advances in conductive polymer research in the 1980srevealed that several families (particularly modified thiophenes) aremuch more stable, and much less reactive toward oxygen. For long-termuse, it is still necessary to package such materials in an anaerobiclight-shielded package, but this is easily done as described below.

Available Principles of Operation

The electronic transport mechanisms for quantum-sized systems withtunnel barriers are either; a) tunneling through localized states (i.e.,resonant tunneling), or; b) hopping (with attendant issues of Coulombblockade); or, c) a combination of both.

Resonant tunneling (as schematically shown in FIGS. 2A-2C) is aconduction mechanism which depends on quantum mechanical tunnelingthrough a quasi-bound quantum-confined state. The simplest embodiment isa quantum well cladded by thin tunnel barriers. Electrons from theemitter of such a structure tunnel through the first barrier into thecentral well region, and then quickly tunnel out. If the central quantumstate is made to be energetically misaligned with the incoming emitterelectrons, such as by a base potential applied to the central quantumwell, the current is dramatically reduced. By this mechanism, atransistor with gain can be produced. Such embodiments have beenextensively demonstrated in semiconductor devices, but not in molecularelectronic structures.

Hopping, or Coulomb blockade, is a different conduction mechanism,wherein the structure can be thought of as a series of small capacitors.If the structure is sufficiently small, the charging energy of thecapacitor, Ec=e² /2C, can be so large that it is energeticallyunfavorable for 2 or more electrons to be on the central terminal; thus,a single electron at a time "hops" through the structure. FIGS. 3A-3Cschematically show this mode of operation, and FIG. 3D shows thecorresponding electrical model.

The hopping mechanism is differentiated from resonant tunneling mainlyby current density; if the collector barrier is sufficiently thin,electrons quickly tunnel through the structure, so Coulomb blockadenever has a chance to take effect; thus, resonant tunneling is themechanism. If the collector barrier is thick and/or high, the electronresides in the central region for a long time, and thus Coulomb blockadeoccurs.

The advantage of resonant tunneling is that high current density andlarge gain are possible. In Coulomb blockade, the ultimate limit of anelectron device (i.e., a single electron device), the current density islow, and it is as yet unclear that large gain can be achieved in such adevice.

According to one embodiment of the present invention there is provided:

A system comprising: a master clock circuit, and an optical outputdriver connected to follow the frequency and phase of said master clockcircuit; a plurality of electronic circuits, on one or more integratedcircuits, wherein plural ones of said electronic circuits includesemiconductor active devices configured as output drivers, and whereinplural ones of said electronic circuits include first and secondconductive contacts thereof and a photoconductive oligomeric structureconnected therebetween; and where light from said optical output driveris optically coupled to multiple ones of said photoconducive oligomericstructures in multiple ones of said circuits.

According to another embodimenmt of the present invention there isprovided: A circuit comprising:

a semiconductor integrated circuit, comprising semiconductor driverdevices, and electrically configured to be connected to first, second,and third contacts;

a first molecular electronics device, electrically configured to receive

a first input signal and to provide a conductivity, between said firstand third contacts, which is modulated in accordance with said firstinput signal;

a second molecular electronics device, electrically configured toreceive a second input signal and to provide a conductivity, betweensaid second and third contacts, which is modulated in accordance withsaid first input signal;

a voltage detection circuit, electrically connected to detect thevoltage of said third contact and provide a corresponding output;

whereby said output of said voltage detection circuit provides a signalwhich is equivalent to a NOR of said first and second input signals.

According to another embodimenmt of the present invention there isprovided:

A picoelectronic device comprising:

first and second conductor chains, each comprising multiple monomorunits having mutually conjugated bonding;

a first barrier region, connected to said first conductor chain, saidfirst barrier having a potential energy for electrons which is lessfavorable than that of said first conductor chain;

a second barrier region, connected to said second conductor chain, saidfirst barrier having a potential energy for electrons which is lessfavorable than that of said second conductor chain;

a well region, connected to said first and second barrier regions, saidwell region having a potential energy for electrons which is morefavorable than that of said first and second barrier regions;

a third barrier region, connected to said well region, said thirdbarrier having a potential energy for electrons which is less favorablethan that of said well;

a third conductor chain, comprising multiple monomor units havingmutually conjugated bonding, and operatively connected to said wellregion through said third barrier region;

whereby changing potentials applied to said third chain can effectmodulation of currents between said first and second chains.

According to another embodimenmt of the present invention there is alsoprovided:

An integrated circuit structure, comprising:

a plurality of transistors;

a plurality of thin-film conductor interconnects, interconnected to formelectronic circuits in a predetermined electrical configuration;

a plurality of pairs of contact pads, connected to said thin-filmconductor interconnects, each adjacent pair of contact pads beinginterconnected being electrically connected only by a conductiveoligomer of a precisely predetermined number of units.

According to another embodimenmt of the present invention there is alsoprovided:

An integrated circuit structure, comprising:

a plurality of transistors;

a plurality of thin-film conductor interconnects, interconnected to formelectronic circuits in a predetermined electrical configuration;

a plurality of pairs of contact pads, connected to said thin-filmconductor interconnects, each adjacent pair of contact pads beingelectrically connected only by a conductive oligomer of a preciselypredetermined number of units.

According to another embodimenmt of the present invention there is alsoprovided:

An integrated circuit structure, comprising:

a plurality of semiconductor transistors;

a plurality of thin-film conductor interconnects, interconnected withsaid semiconductor transistors to form electronic circuits in apredetermined electrical configuration;

a plurality of pairs of contact pads, connected to said thin-filmconductor interconnects;

a plurality of molecular electronic active devices, each including aconductive oligomer connecting one of said contact pads, and abarrier-well-barrier structure connected to modulate the conductivity ofsaid conductive oligomer.

According to another embodimenmt of the present invention there is alsoprovided:

An integrated circuit structure, comprising:

a plurality of transistors;

a plurality of thin-film conductor interconnects, interconnected to formelectronic circuits in a predetermined electrical configuration;

a plurality of pairs of contact pads, connected to said thin-filmconductor interconnects, each adjacent pair of contact pads including afirst pad of a first conductive material and a second pad of a secondconductive material, and being electrically connected only by aconductive oligomer of a precisely predetermined number of units.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1A shows a resonant tunnelling device in the on-state, and FIG. 1Bshows the device of FIG. 1A in the off-state (after the potential of thebase has been shifted).

FIG. 2A shows a resonant tunnelling device in the on-state, and FIG. 2Bshows the device of FIG. 2A in the off-state (after wave-functionmodulation has changed the height of the second barrier).

FIG. 3A shows an electron-hopping device with its well empty. FIG. 3Bshows how, if the well of the device of FIG. 3A contains an excesselectron, the likelihood of another electron hopping into that well isgreatly reduced by the "Coulomb blockade" effect. FIG. 3C shows how, ifthe well of the device of FIG. 3A contains an excess electron, thatelectron can hop out of the well. FIG. 3D shows amacroscopic-quasi-equivalent circuit diagram of the device of FIG. 3A.

FIG. 4A shows the spatial variation of conduction band (CB) and valenceband (VB) energy levels across a first example monomer unit which canform conjugated conductive polymer structures. FIG. 4B shows the spatialvariation of conduction band (CB) and valence band (VB) energy levelsacross a second example monomer unit which can form conjugatedconductive polymer structures. FIG. 4C shows how, when two such monomerunits are chemically combined, the resulting dimer structure has a bandstructure which produces a barrier-well-barrier-well-barrier profile.

FIGS. 5A and 5B are a corresponding pair of drawings of two states ofoperation of a novel molecular electronic device.

FIG. 5A shows the ON state. In this state an energy level in the wellregion is aligned with the energy level of incoming electrons, and thusresonant tunnelling can occur, to produce a net flow of electrons fromthe "emitter" terminal through to the "collector" terminal. FIG. 5Bshows the OFF state. In this state a different potential has beeninduced at the "base" terminal. This induced potential propagates,through the chain X, to change to energy levels in the well region. As aresult of this change, no energy level in the well region is alignedwith the energy level of incoming electrons, and thus resonanttunnelling does not occur, and therefore current flow does not occurbetween the "emitter" terminal and the "collector" terminal.

FIG. 6 shows a sample structure for realizing the coupling between themodulator side chain and the energy levels of a well in the conductivepolymer chain.

FIGS. 7A, 7B, and 7C show a sequence of steps to produce self-aligneddeposition from solution of selective contacts between oligomer ends andmetal contact pads.

FIGS. 8A, 8B, and 8C show a sequence of steps to producelength-selective deposition from solution of only those conductiveoligomers which have a predetermined length L.

FIG. 9 shows how a molecular electronic active device is interconnectedwith semiconductor devices, using first and second metal contact pads M1and M2.

FIG. 10 shows a structure in which a conductive polymer providesself-aligned length-selective connection of metal contact pads M1 andM2, to connect semiconductor devices together.

FIG. 11 shows a NOR gate using molecular electronic devices.

FIG. 12 shows a structure in which length-selective oligomericinterconnects provide interconnection and electrical orientation ofmolecular electronic devices.

FIGS. 13A, 13B, and 13C show three alternative structures for inputtinga signal into the modulator side-chain (in a device like that of FIGS.5A and 5B):

FIG. 13A shows how photonic input provides a potential change at aphotosensitive end-group on the modulator chain;

FIG. 13B shows how electrical input, from a metal or semiconductorcontact, provides a direct potential change at an end-group on themodulator chain; and

FIG. 13C shows how ionic population shifts, in a micro-localized medium,provides a potential change on the modulator chain.

FIGS. 14A, 14B, 14C, 14D1, 14E1, and 14F1 show a first self-alignedprocess for fabrication of contacts of two different materials, withsublithographic spacing, in an integrated circuit fabrication process.

FIGS. 14A, 14B, 14C, 14D2, 14E2, and 14F2 show a second self-alignedprocess for fabrication of contacts of two different materials, withsublithographic spacing, in an integrated circuit fabrication process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features but not to others.

Oligomeric Interconnects

The oligomers, in the presently preferred embodiment, will be thiopheneand thiophene-ethynylene oligomers of 100 Å in length (two 50A chainsseparated by a barrier-well-barrier structure). Thiophenes exhibitconductivities, in the bulk, of 100-200 Ω⁻¹ cm⁻¹. The ends of theoligomers will be functionalized with a thiol group on one end and acarboxyl groups on the other. Thiophene units can be readilydeprotonated using bases like LDA, butyllithium, or alkyllithiums;hence, end group functionalization such as an organolithium can beeasily accomplished. (Since lithium is at the top of theelectropositivity scale, one can transmetalate to almost all othermetals, or react with almost any electrophile. For example, a thiopheneend group can be lithiated and converted to a (1) thiol for adhesion toAu surface, (2) for adhesion to Pd surfaces, (3) carboxylated foradhesion to oxide surfaces, (4) transmetalated and cross-coupled tobipyridyls for adhesion to Fe, etc.) The thiol group will selectivelyattach to a gold metal pad, and the carboxyl group to an aluminum pad,selectively for pad spacing of less than or equal to 100 Å. The padswill be defined by E-beam lithography and liftoff.

Synthesis of Controlled-Length Oligomers

In the presently preferred embodiment, thiophene units are used as theoptimal organic subunits for controlled oligomer growth. This is because(1) oligo-or poly-thiophenes exhibit conductivities, in the bulk, of100-200 Ω⁻¹ cm⁻¹, (2) The oligomers are air and light stable and theycan be handled with no exotic precautions, (3) the alkyl groups affordmaterials that are freely soluble in organic solvents with no loss inthe conductivities due to distortions in planarity of the contiguousp-backbone, and (4) thiophene units can be readily deprotonated usingbases like LDA or butyllithium; hence, end group functionalization canbe easily accomplished.

The synthesis of thiophene-ethynylene systems is expected to allow therapid chain growth of conjugated oligomers (molecular wires). Simply,monomer will be converted to dimer, dimer to tetramer, tetramer tooctamer, octamer to 16-mer, 16-mer to 32-mer, etc. In this way, therewould be a rapid growth of the molecular chains. The synthetic routewith the progress to date is shown in Scheme II.

Selective Attachment to Electrical Contact Pads

The present application provides an automatic attachment process, inwhich we take a conductive polymer, and attach ("functionalize") ontothe ends a compound that can selectively attach to a metal probe.Numerous examples of these "self-assembling" compound/metal pairs areknown; for example, n-alkanethiols onto Au, isonitrile onto Pt, andalkanecarboxylic acid onto aluminum oxide.¹¹ This is in essence aconducting string with sticky ends, which could bridge a gap betweenmetallic contacts (of the selective metal). By fabricating (by E-beam orSTM) closely-spaced metallic contacts, the molecular wire can bespontaneously deposited from solution. Note that if the molecular wireis synthesized with different end groups on opposing ends, the polarityof the attachment can be defined. The specific contact resistance ofsuch an ohmic contact is not yet precisely known, though the large valueof the bond energies imply this may not be a problem; for the organicthiolates and Au, this is 40-45 kcal/mole. These "selective-attachmentconducting polymers" (specifically, conjugated organic oligomers withfunctionalized selective attachment termini) provide a technique forspontaneously generating contacts between metallic endpoints, at themolecular scale (10-100 Å).

An advantageous application is for simple self-aligned interconnects;given a device with metal A on one terminal (for example, collector),and a second device with metal B on one terminal (for example, base), amolecular wire with end groups A' and B' (which attach selectively to Aand B, respectively) can bind selectively to make an interconnect,without a lithography step. Though we will see that interconnects arenot the most important application, this spontaneous "lock-and-key"concept is the basic ingredient. Also note that this process is, to adegree, length dependent. Interconnections of contacts separated bylonger than the designed molecular wire length are prevented. Animportant technology issue is the nuisance of unwanted binding of thepolymers other than at the terminal ends. It appears that this concerncan be solved for large metallic contacts (other than simple bindingposts) by either selective exposure of the metal (i.e., in the simplestcase by via holes) through an insulating overlayer coating at only thecontact points desired, or by post-attachment scavenging of the unwanteddangling molecular wires.

Using synthesis similar to that outlined above, functional groups willbe inserted into the conducting oligomer chain. An organic quantum well(more properly, quantum dots) with p- conjugated benzene rings and --O--barriers will be synthesized. A single thiophene unit would serve as theembedded quantum dot, cladded with --O-- tunnel barriers. The thiopheneunit will also be conjugated to another tunnel barrier, --S--, whichforms the base. This barrier will have attached to it another thiopheneunit, another barrier (--O--), and an oligomer chain with afunctionalized end. Voltage input to the base via the conductingoligomer will modify the energy structure of the quantum well, thusmodulating the current. Each of the 3 terminals (emitter with a thiolgroup, collector with a carboxyl group, and base with a diarylphosphinegroup) will attach spontaneously and selectively from solution to themetal pads (gold emitter, aluminum collector, and palladium base) in aunique configuration. This is a single molecule organic resonanttunneling (or Coulomb blockade) structure, with exact fabricationtolerance.

Optional Side Groups

The additional of a stabilizing side group, such as ethyl, can help tomaintain the solubility of a large oligomer.

Option for Stabilizing Conductor Strands

One advantage of polythiophenes is that alkoxy (--OR) sidegroups (suchas --OCH₃ or --OCH2CH₃) add environmental stability to the polythiophenechain. Thus, use of such sidegroups may provide further advantages inthe presently preferred embodiment.

To provide additional in situ stability, it is also possible to addside-groups which will have affinity for a known substrate. Thus, forexample, if the polymer conductor is to connect two metal contactsacross an oxide surface (e.g. SiO₂), a side group with a mild affinityfor oxide can be added. An alkyl chain can be added without harming theconductivity of thiophenes, and a hydroxyl end group on the alkyl chainwill form weak hydrogen bonding to the silicon dioxide.

Option for Fusing Conductor Strands

If one wanted small cross-linked packets of molecules that could bridgethe gap between two surface, there remains the possibility of using thesame self-assembly techniques. The metal surfaces would be made largeenough that several hundred molecules could bridge the gap. Once theoligomeric units are bridging the gap and attached via the Z groups tothe metal contacts, excess oligomers could be washed away. Then, theoligomers, upon exposure to an oxidant or Lewis acid (i.e. I₂ or FeCI₃),would probably cross-link in place to form a semi-bulk-like material(which would still have a thickness only on the order of only a fewhundred molecules thick). These extended oligomers are quite prone tosuch cross-linking upon mild acid exposure as described in Skotheim's"Handbook of Conducting Polymers" as well as in our "Macromolecules"reference above.

Device Embodiments

General Considerations Unsuccessful Prior Designs

Up to now, researchers in organic device development have been pursuingembodiments of electron devices such as the so-called "Aviram switch".¹²In this conceptual specification, a delocalized conducting polymer chainhas a non-conducting polymer fixed to it at 90° via a non-conjugateds-bonded network. This provides a simple crossbar switch. Suchstructures are now being realized; using similar methods as describedabove, two orthogonally fused oligomers (28 and 29) shown below weresynthesized in the USC laboratory. Even these complex structure werefreely soluble in organic solvents, due to the presence of alkyl groupsat several of the thiophene units. Each of these structures have beensuggested for future molecular electronic devices.¹³

In fact, the following orthogonally fused oligomers for an Aviram switchhave actually been fabricated at the University of South Carolina:##STR1##

A straightforward application of the selective attachment technologydescribed allows us to "tag" appropriately each of the termini. Fourappropriately placed nanolithographic probes allow us to lay down thetagged molecule using a lock-and-key strategy, so that we can determineif such structures have the desired electronic properties; i.e., does anAviram switch work, and does it have any gain?

The Aviram switch is one approach to realizing a molecular functionalelectron device. However, the present application discloses analternative approach to realizing a molecular functional electron devicethat is a straightforward extension of the conducting oligomer workdescribed above, and has an active region that has already beendemonstrated (though only by optical characterization). Electrically,the approach is highly analogous to present solid state devices, andshould provide a direct path to realizing useful electronic devices.Using synthesis similar to that outlined above, it has been demonstratedthat functional groups can be inserted into the conducting oligomerchain of a simple wire to create complex bandgap engineered (in thiscase, molecular orbital engineered) structures, such as quantum wells ortunnel barriers. An example of a molecular quantum well isphthalocyanine,¹⁴ which exhibits 4 quantum wells, with barrier heightsof only a few 10s of kT_(room).

Very recently, organic quantum wells (more properly, quantum dots) withp-conjugated benzene rings and various barriers (--S--, --O--, and --CH₂--) have been reported.¹⁵ Optical measurements exhibit shifts in theoptical absorption peak corresponding to appropriately changing eitherthe size of the "dot" (i.e., changing the number of benzene units in thedot region) or the barrier height (using different barrier molecules).Thus, one can achieve the same type of bandgap engineered (here,molecular orbital engineered) electronic structures analogous to thosethat exist in quantum solid state heterojunction devices. Examples ofthe type of molecular tunnel barriers and quantum wells that havealready been realized are illustrated in FIGS. 4A-4C.

FIGS. 4A-4C show how the monomer components of a polymer can provideelectronic operation analogous to solid state heterojunction quantumwells and tunnel barriers (see reference 18). The schematic conductionband (CB) and valence band (VB) are diagrammed.

According to the teachings of the present application, such organicmolecular bandgap engineering is used to realize electronic structuresby conjugating a selective-attachment wire to the designed quantum wellactive region; for example, the molecular equivalent of a resonanttunneling structure is achieved by conjugating the conducting oligomersonto the barrier ends of an organic quantum well (e.g., the example onthe far right of FIGS. 4A-4C), then attaching this resultant moleculebetween nanofabricated contacts. This is a single molecule organicresonant tunneling (or Coulomb blockade) structure, with exactfabrication tolerance. The organic embodiment eliminates the problem ofdimensional tolerance found in the heterojunction solid state version,since the synthesis in the molecular case is atomically discrete andunique, whereas self-limiting mechanisms are difficult to achieve in thesolid state (i.e., the molecular approach achieves the goal of atomiclayer epitaxy). Whether one has resonant tunneling or single electroncharging depends on the details of the energy band (i.e., molecularorbital) structure; a priori, it would seem that high current densityresonant tunneling would be preferable. It is expected that the largeorbital energies will ensure room temperature operation. In the proposedstructure, a single thiophene unit would serve as the embedded quantumdot, cladded with --O-- or --S-- tunnel barriers. Onto the barriers, theconducting oligomers with selective attachment ends would be conjugated.Modeling would help identify sufficiently low tunnel barriers for largecurrent drive. To our knowledge, this approach to molecular quantumdevices and the ability to fabricate selective contacts has neither beenreported or proposed.

This two-terminal structure is a major milestone in this technology, asit combines the three key ingredients of the technology; 1) synthesis ofconducting oligomers with selective attachment end groups; 2)conjugation of these oligomers onto a designed active region; and, 3)spontaneous attachment of a single molecule electron device ontonanofabricated contacts. This is an important intermediary diagnosticstep toward achieving the final goal, three (or more) terminal organictransistors with gain. The fabrication of these transistors will requirea non-trivial design of the "active region" molecule through modeling.

A molecular transistor that exhibits gain will have the same requirementthat solid state electron devices have; that is, gain arising frommodulation of non-equilibrium injected carriers by a second distributionof isolated carriers. In the molecular embodiment, it is quicklyrealized that the solid state analogies can no longer hold, sinceCoulomb blockade will not allow isolated controlling carriers to existin the active region. However, there is an alternative, which has ademonstrated proof-of-principle (though so far only in opticalembodiments). This alternative uses an active region structure thatoperates similar to a hot electron transistor (HET). The requirementswould be that the quantum well active region (base), which can carry alarge transport current density, be conjugated via s-bonds to a remotecharge transfer molecule. The properties of this molecule is designed sothat the base molecular orbital energy can change dependent on thecharge state of the remote (a few bond lengths away) section of themolecule; i.e., gating via deformation of molecular orbitals.

Several methods are disclosed for coupling an input signal into themodulator side-chain of an oligomeric active device. The simplestconnects the side-chain to an electrical contact. Another disclosedmethod uses a photosensitive compound to generate a voltage shift underillumination. Another disclosed method uses direct coupling of themodulator side-chain (the "base") of one active device to the outputchain (the "collector") of another.

A common example of a molecule having photovoltaic properties isbacteriorhodopsin, which exhibits photoisomerization between two stablemolecular orbital configurations. Specifically, incident light in a dyemolecule produces a singlet excited state, which transfers an electronto a remote bacteriopheophytin molecule. In this case, the absorptionlevel due to the molecular orbital reconfiguration changes by ˜0.35 eV.(For a comparison to solid state systems, this level change is almost asgreat as the entire (direct gap) conduction band offset of a GaAs/Al_(x)Ga_(1-x) As quantum well).

Bacteriorhodopsin and rhodopsin are not themselves practical candidatesfor use with molecular electronic devices in a quasi-thin-filmenvironment. However, many other classes of molecules provide usefulphotosynthetic reaction centers.¹⁶ Some attractive candidate species arelisted by Pietoni in 114 J.A.C.S. 2734 (1992),¹⁷ and by J. -M. Lehn,1991 J. CHEM. SOC. 1179, both of which are hereby incorporated byreference.

Choice of Oligomeric Conductor

Thiophene is the preferred conductor. Thiophene has been demonstrated toprovide adequate conductivity and stability, and also permits convenientmodification with sidegroups and endgroups as detailed elsewhere herein.However, alternative conductive oligomers can be used if they provideadequate conductivity and stability.

In general, the thiophene monomeric units provide their bestconductivity between ring positions 2 and 5 (the two positions adjacentto the sulfur atom). Thus, the specific examples given below use thisconfiguration. Adjacent thiophene monomers preferably have opposite (butapproximately coplanar) orientations. This permits the oligomer to takea more extended trans-old configuration.¹⁸

Barrier Design at Modulator Input

FIGS. 5A and 5B are a corresponding pair of drawings of two states ofoperation of a novel molecular electronic device.

FIG. 5A shows the ON state. In this state an energy level in the wellregion is aligned with the energy level of incoming electrons, and thusresonant tunnelling can occur, to produce a net flow of electrons fromthe "emitter" terminal through to the "collector" terminal.

FIG. 5B shows the OFF state. In this state a different potential hasbeen induced at the "base" terminal. This induced potential propagates,through the chain X, to change to energy levels in the well region. As aresult of this change, no energy level in the well region is alignedwith the energy level of incoming electrons, and thus resonanttunnelling does not occur, and therefore current flow does not occurbetween the "emitter" terminal and the "collector" terminal.

FIG. 6 shows a sample structure for realizing the coupling between themodulator side chain and the energy levels of a well in the conductivepolymer chain. The trisubstituted emitter/collector/base unit can beprepared as follows. The central 1,3,5-benzenetrimethanol is a knowncompound¹⁹ and can serve as the junction or "W" unit represented inFIGS. 5A and 5B. Treatment with excess p-toluenesulfonyl chloride("TsCI" or "tosyl chloride") in pyridine would rapidly afford the1,3,5-benzenetris(hydroxymethylenetosylate). An oligothiophene of 16units with a metal binding group Z (see below for a description of Z)and n-butyl groups attached would be fully soluble based on our previousexperience with thiophene oligomer synthesis²⁰. Treatment of tiethiophene oligomer with LDA would afford the terminal thiophene anionwhich could be reacted with an excess of the1,3,5-benzenetris(hydroxymethylenetosylate) to form the monosubstitutedcore. Likewise, treatment of the monosubstituted core with a secondoligomer anion containing the same or a different metal binding group Z'would afford the disubstituted core. Undoubtedly, a separation would beneeded here; however, sterics interactions of the incoming nucleophileshould favor the formation of the disubstituted core shown. Againtreatment of the disubstituted core with a third oligomeric anioncontaining the same or a different metal binding group Z" would affordthe desired trisubstituted emitter/collector/base unit shown. Themethylene units (CH₂ groups) serve as the large barrier units. Thephenyl group serves as the low barrier unit. It is well-known thatconversion of a benzenoid structure to the quinoidal form for conductionis far more difficult than conversion of the thiophene units to theircorresponding quinoidal forms. This trend is easily seen in the band-gapdifferences for polyphenylene and polythiophene at 3.2 eV and 1.9 eV,respectively²¹.

The thiophene oligomers could be synthesized according to methodsdescribed previously for soluble thiophene oligomer synthesis. The onechain containing the thiophene/phenylene unit could be synthesized asshown below. These metal-catalyzed coupling are quite standard inorganic synthesis and our three previous papers listed above outlinethese procedures. Note that "Me" stands for methyl of CH₃ group.Similarly, the thiophene-phenylene dimer could be prepared as follows.##STR4##

Choice of Modulation Input Source

Several methods are disclosed for coupling an input signal into themodulator side-chain of an oligomeric active device. The simplestconnects the side-chain to an electrical contact. Another disclosedmethod uses a photosensitive compound to generate a voltage shift underillumination. Another disclosed method uses direct coupling of themodulator side-chain (the "base") of one active device to the outputchain (the "collector") of another. Yet another disclosed method usesion pumping in a confined gel (as schematically shown in FIG. 13C, toeffect an ionic coupling of the modulator side-chain (the "base") of oneactive device to the output chain (the "collector") of another.

Off-State Impedance

For some applications, a driving need is to modulate the device currentdown to zero in the off state. For such applications, the device ispreferably modified to include multiple modulated gain stages in series.This can be implemented as a molecular structure which includes abarrier-well.. barrier .well--barrier structure.

Tunnelling versus Hopping Operation

The electronic transport mechanisms for quantum-sized systems withtunnel barriers are either; a) tunneling through localized states (i.e.,resonant tunneling), or; b) hopping (with attendant issues of Coulombblockade); or, c) a combination of both.

Resonant tunneling (as schematically shown in FIGS. 2A-2C) is aconduction mechanism which depends on quantum mechanical tunnelingthrough a quasi-bound quantum-confined state. The simplest embodiment isa quantum well cladded by thin tunnel barriers. Electrons from theemitter of such a structure tunnel through the first barrier into thecentral well region, and then quickly tunnel out. If the central quantumstate is made to be energetically misaligned with the incoming emitterelectrons, such as by a base potential applied to the central quantumwell, the current is dramatically reduced. By this mechanism, atransistor with gain can be produced. Such embodiments have beenextensively demonstrated in semiconductor devices, but not in molecularelectronic structures.

Hopping, or Coulomb blockade, is a different conduction mechanism,wherein the structure can be thought of as a series of small capacitors.If the structure is sufficiently small, the charging energy of thecapacitor, Ec=e² /2C, can be so large that it is energeticallyunfavorable for 2 or more electrons to be on the central terminal; thus,a single electron at a time "hops" through the structure. FIGS. 3A-3Cschematically show this mode of operation, and FIG. 3D shows thecorresponding electrical model.

The hopping mechanism is differentiated from resonant tunneling mainlyby current density; if the collector barrier is sufficiently thin,electrons quickly tunnel through the structure, so Coulomb blockadenever has a chance to take effect; thus, resonant tunneling is themechanism. If the collector barrier is thick and/or high, the electronresides in the central region for a long time, and thus Coulomb blockadeoccurs.

The advantage of resonant tunneling is that high current density andlarge gain are possible. In Coulomb blockade, the ultimate limit of anelectron device (i.e., a single electron device), the current density islow, and it is as yet unclear that large gain can be achieved in such adevice.

First Preferred Device Embodiment

This sample embodiment provides selective attachment of organic wires.

The oligomers will be thiophene and thiophene-ethynylene oligomers of100 Å in length. Thiophenes exhibit conductivities, in the bulk, of100-200 Ω⁻¹ cm⁻¹. The ends of the oligomers will be functionalized witha thiol group on one end and a carboxyl groups on the other. Thiopheneunits can be readily deprotonated using bases like LDA, butyllithium, oralkyllithiums; hence, end group functionalization such as anorganolithium can be easily accomplished. (Since lithium is at the topof the electropositivity scale, one can transmetalate to almost allother metals, or react with almost any electrophile. For example, athiophene end group can be lithiated and converted to a (1) thiol foradhesion to Au surface, (2) for adhesion to Pd surfaces, (3)carboxylated for adhesion to oxide surfaces, (4) transmetalated analcross-coupled to bipyridyls for adhesion to Fe, etc.) The thiol groupwill selectively attach to a gold metal pad, and the carboxyl group toan aluminum pad, selectively for pad spacing of less than or equal to100 Å. The pads will be defined by E-beam lithography and liftoff.

Second Preferred Device Embodiment

This second sample embodiment provides a molecular active device whichemploys resonant tunneling.

Using synthesis similar to that outlined above, functional groups willbe inserted into the conducting oligomer chain. An organic quantum well(more properly, quantum dots) with p- conjugated benzene rings and --O--barriers will be synthesized. A single thiophene unit would serve as theembedded quantum dot, cladded with --O-- tunnel barriers. The thiopheneunit will also be conjugated to another tunnel barrier, --S--, whichforms the base. This barrier will have attached to it another thiopheneunit, another barrier (--O--), and an oligomer chain with afunctionalized end. Voltage input to the base via the conductingoligomer will modify the energy structure of the quantum well, thusmodulating the current. Each of the 3 terminals (emitter with a thiolgroup, collector with a carboxyl group, and base with a diarylphosphinegroup) will attach spontaneously and selectively from solution to themetal pads (gold emitter, aluminum collector, and palladium base) in aunique configuration. This is a single molecule organic resonanttunneling (or Coulomb blockade) structure, with exact fabricationtolerance.

Third Preferred Device Embodiment

This sample embodiment provides a molecular resonant tunneling devicewhich can be activated by optical input.

Using synthesis methods similar to that outlined above, functionalgroups are inserted into the conducting oligomer chain. An organicquantum well (more properly, quantum dots) with conjugated benzene ringsand --O-- barriers will be synthesized. A single thiophene unit wouldserve as the embedded quantum dot, cladded with --O-- tunnel barriers.The thiophene unit will also be conjugated to another tunnel barrier,--S--, which forms the base. This barrier will have attached to itanother thiophene unit, another barrier (--O--), and a dye molecule suchas bacteriorhodopsin. Photon input to the base will transfer an electronto the base thiophene unit, which will modify the energy structure ofthe quantum well, thus modulating the current. Only 2 terminals (emitterwith a thiol group, collector with a carboxyl group) will attachspontaneously and selectively from solution to the metal pads (goldemitter, aluminum collector) in a unique configuration. The base willhang freely.

Module Embodiments

Assembly of the disclosed molecular electronic devices into a completeintegrated circuit bears some significant differences from the assemblyand packaging of a conventional semiconductor integrated circuit.

Combination of Molecular and Semiconductor Active Devices

A major problem for most proposed nanoscale device technologies isfanout; there will always be a need for intermediate current drive(amplifiers) in few-electron systems. The solution is easy toincorporate in this technology. A conventional transistor amplifierlayer would be the starting substrate. After coating with an insulatinglayer, the metal interconnect and molecular layers are fabricated ontop, with input to the amplifiers made by via holes (for example, thepost in FIG. 12). This allows one to derive or input an internal signalanywhere in the array.

Self-Aligned Processes for Preparing Metal Contacts

with Nano-Scale Separation

In such a hybrid structure, one important consideration is how toprepare contacts, within a semiconductor fabrication process, which willhave small enough dimensions to take advantage of the very small scaleof the molecular devices. One option for doing this is E-beam directwrite, and a probably long-term alternative is masked ion beamlithography.

However, it can also be advantageous to prepare gaps which havesublithographic spacing. The following process flow is an example of aself-aligned process for achieveing this.

1) (See FIG. 14A) Start with Si substrate; oxidize (thick) SiO₂ layer;put on nitride layer.

2) (See FIG. 14B) define "thin sidewall" by

i) CVD Si

ii) pattern small Si squares

iii) recrystallize Si (if necessary)

iv) 100A. SiO₂ anneal

v) ion etch briefly (˜150 Å)

vi) RIE Si

3) (See FIG. 14C) remove sidewall except for 1 side (photoresist & SiO₂etch)

4) (See FIG. 14D1) Define contact pads (optical); evaporate metals 1 and2

5) Etch SiO₂ --lifts off metal to create a 100 Å break.

6) (See FIG. 14E1) Do same "sidewall" trick to create a 100 Å wallacross pads

7) (See FIG. 14F1) evaporate some insulator (e.g. CaF₂); etch SiO₂ (liftoff CaF₂); left with exposed metal

8) Add the polymers.

This process can also be varied with alternative steps as follows:

4') (See FIG. 14D2) Pattern nitride in the following configuration(using optical photoresist and plasma etch):

5') (See FIG. 14E2)

i) Spin on Photoresist ("PR")

ii) Partially etch the PR in an O₂ plasma until the SiO₂ ridge sticksup.

iii) Etch SiO₂ (under RIE conditions)

iv) Etch SiO₂ with undercut (e.g. wet etch)

v) Strip PR. These steps have produced a nitride bridge.

6') (See FIG. 14F2) Now evaporate metals 1 and 2 at different angles.The angle deposition provides a reduction of the 1μ gap to about 100 Å(depending on the specific thicknesses and angles used).

Active Device Types

The preferred active device architecture is as shown in FIGS. 5A and 5B.However, the alternative principles of FIGS. 2 or 3 may alternatively beapplied instead.

The semiconductor active devices are preferably MOSFETs, for their highinput impedance and low power draw. Of course, these are preferablycombined with conventional ESD diodes at I/O connections.

Passivation

Of course, the completed module is hermetically sealed from atmosphericoxygen. The simplest way to do this is with a bubble seal which enclosesan inert atmosphere (nitrogen or argon). A simple epoxy seal will do agood job of this.

Alternatively, a passivation layer over the active devices is used toscavenge any small amounts of oxidant from the sealed environment.

Examples of Novel Circuit Implementations

Assuming that we can create active devices with "tagged" terminals, weonly need to (nanolithographically) define a connection pattern as thefirst step. We do not assume any specific nanolithographic tool(although it is preferable that such a step will be parallel, such asMIBL, X-ray, or masked E-beam). This initial step defines an aligningmatrix for the molecules. We then attach (from solution) the synthesizeddevice(s), in a lock-and-key strategy. Let us consider how to createsimple gates. In some cases, we can work with just one "polarity" oftagging. For example, a NOR gate would only need a single polarity, asshown in FIG. 11. Here the organic transistor terminals (collector,emitter, and base; or, C, E, B) are tagged with selective attachment tometals M1, M2, and M3, respectively. After fabrication of the metals inthe diagrammed pattern, the tagged molecule spontaneously andselectively arrange to form the gate as shown.

NOR Gate

FIG. 11 is a schematic of a molecular NOR gate. The shaded areas are theinitially defined metals M1, M2, and M3. The triangles represent thespontaneously assembled molecular transistors. For this gate, thepolarity of the tagging is {E:M2}, {B:M3}, and {C:M1}, where E=emitter,B=base, C=collector. The output can either be metal wire or a M1 taggedoligomer.

For complex designs, the metal (or otherwise) attachment pads need notbe connected externally, and can serve as binding post attachment postsfor transistor-transistor "soldering", as illustrated in FIG. 12, whichis a schematic of a molecular binding post arrangement, for arbitrary{E,B,C} connections. The shaded area is metal M1, and the trianglesrepresent the molecular transistors with attachment ends L1.

This allows us to define a topology, unaligned (indeed, as the initialstep), of active device interconnections without external contacts. Somemore complex circuits will require a number of different polarities andtypes of active elements. The fabrication for such a system wouldrequire sequential spontaneous absorptions of each type, from solution.The only constraint is that each polarity have a unique contactgeometry, and that each successive step does not disturb the subsequentstages (i.e., process integration. While this is clearly not a trivialengineering feat, there are no fundamental limitations yet identified.)Thus, the circuit configuration is determined in the reverse order thanthat of conventional fabrication; the active devices spontaneouslyself-assemble onto the interconnects. Note that the interconnects thatrun long distance are few; most are simply attachment sites to connectemitter to base, etc.; thus, metal interconnects will not define thelower limit of device scaling, with a design that mixes metal andconductive oligomer contacts. This approach appears to optimally packdevices.

Multi-Input (Quasi-Analog) Gates

By including multiple modulator chains at separate inputs on a conductorchain, conductivity can be modulated independently by several separateinputs. Such quasi-analog gates may be useful for applications such aspattern recognition.

Novel System Applications

The device and circuits described lend themselves well to systems inwhich an array of molecular circuits can advantageously be formed, in awholly self-aligned manner, by a sequence of batch-processed steps.

Clock Distribution

A significant issue for any large integrated system is clockdistribution and skew. As was previously discussed, there exists a classof photoisomerization molecules which could be directly utilized here.At nodes where one wishes to distribute a global clock that cannot bedistributed by the underlying amplifier array, a photosensitive electrontransfer molecule would be attached to the base, thus supplying anoptically generated clock with no skew. The ability to conjugate avariety of different photosensitive electron transfer molecules on thesame basic transistor structure could provide the capability of amultiphase clock.

Large Neural Networks

Neural networks are an extremely attractive technology which hasattracted increasing interest over the last 30 years. A great advantageof the neural network architecture is that it is not necessary todirectly specify every connection, nor to have direct access to everystored bit of dam. However, this imposes a correlative difficulty inscaling to large sizes: the peripheral access circuits become lessclosely coupled to the "interior" of a neural net as the size of thearray increases.

The disclosed novel architectures, by providing self-asemblingelectronic devices with electrical access available at any point, offerthe potential to provide a large advance in neural networkarchitectures.

Holographic Storage

The disclosed innovative technology provides a new way to economicallyfabricate arrays with very high density. Therefore, one particularlyattractive application is in direct or internal addressing ofholographic memory.

Holographic memory exhibits many attractive characteristics, but itsdifficulties have prevented it from making any serious threat to becomeother than marginal. One contribution potentially available from thedisclosed inventions is to permit direct access to modify or overlayholographically-organized date.

Sample Processing Sequence

We now outline the detailed synthetic organometallic approach torealizing these conjugated organic oligomers. The oligomers will bewell-defined, homogeneous materials that are fully characterized fromthe structural standpoint. No range of molecular weight compounds ormolecular lengths will exist. These will be chemically pure (>97%)materials (initially thiophene and thiophene-ethynylene oligomers), oflengths that can be determined to ˜0.5 Å. The oligomers will range from10 Å to at least 100 Å in length. In the undoped form, they will be airand light stable for at least 24 hours, and stable for months or yearsin an inert (N₂) atmosphere in the absence of light.

The ends of the oligomers will be appropriately functionalized withvarious organic groups that are known to adhere to specificnanolithographic probe surfaces. These functionalities may be thought ofas molecular "alligator clips". Once these functionalized oligomers aresynthesized, we can electrically characterize a single molecularoligomer by "stringing" it between nanofabricated ˜100 Å spacing metalcontacts. Simultaneously, we can structurally characterize the adheredoligomer, in situ, by STM. At present, conjugated oligomers with maximumlengths of ˜50 Å have been fabricated. These lengths need to be extendedto ˜100 Å to coincide with long term nanolithographic requirements.Thus, the synthesis effort will have two parallel paths; extend theconjugated oligomer length to 100 Å; and conjugate selective attachmentstructures onto existing, shorter lengths to gain synthesis experienceof compatibility of processes.

Candidate attachment end groups are thiol, carboxyl, pyridyl, orphosphine groups. The electronic properties of the attachment structuresare key to the synthesis direction. The fabrication/characterizationeffort simultaneously will be characterizing the attachment end groups,by attachment onto nanofabricated structures, and subsequent STM. Theseresults will provide information about the binding energies andproperties of the various proposed attachment structures.

The synthesis of homogeneous conjugated oligomers beyond 50 Å in lengthhas never before been accomplished. Two groups recently reported theformation of 40 Å oligomers; however, there were two primarydeficiencies in those systems that would make it difficult for theirincorporation into nanolithographic architectures.²² First, theconjugated polyolefins are not stable to air and light at ambienttemperatures for even short time periods (<30 min half lives). Second,and even more importantly, the syntheses do not lend themselves to thepreparation terminally functionalized oligomers which are necessary ifthe molecules are to bind to probe surfaces. The group of Prof. J. Tourrecently described the synthesis of thiophene oligomers from 3 Å to 30Å. The synthesis and reagents necessary for the synthesis are shown asfollows in Scheme I.²³

In this process schema, the reagents are:

(a) n-BuLi, TMEDA²⁴ ; TMSCl²⁵

(b) LDA²⁶ ; TMSCI

(c) n-BuLi; I₂

(d) Mg; 5, Cl₂ Ni(dppp²⁷)

(e) t-BuLi; B(O-i-Pr)3; H₃ O+

(f) Pd(PPh₃)₄, Na₂ CO₃, H₂ O

(g) Br₂

(h) n-BuLi; H₂ O

(i) MeMgBr, Cl₂ Ni (dppp)

(j) HgO, I₂

(k) LDA; R₃ SnCl²⁸

(l) LDA; I₂

(m) Pd(PPh₃)₄, toluene

(n) t-BuLi; I₂

(o) Mg; 16, Cl₂ Ni(dppp)

(p) Mg; 3, Cl₂ Ni(dppp).

In the presently preferred embodiment, thiophene units are the optimalorganic subunits for controlled oligomer growth. This is because (1)oligo- or poly-thiophenes exhibit conductivities, in the bulk, of100-200 Ω⁻¹ cm⁻¹, (2) The oligomers are air and light stable and theycan be handled with no exotic precautions, (3) the alkyl groups affordmaterials that are freely soluble in organic solvents with no loss inthe conductivities due to distortions in planarity of the contiguousp-backbone, and (4) thiophene units can be readily deprotonated usingbases like LDA or butyllithium; hence, end group functionalization canbe easily accomplished.

Scheme I shows how trimethylsilyl groups have beech demonstrated in useto cap the ends of thiophene oligomers. The trimethylsilyl groupsallowed control of the oligomer growth at each stage in the synthesis,and may provide a handle for future chemoselective modifications.²⁹

The synthesis of thiophene-ethynylene systems is expected to allow therapid chain growth of conjugated oligomers (molecular wires). Simply,monomer will be converted to dimer, dimer to tetramer, tetramer tooctamer, octamer to 16-mer, 16-mer to 32-mer, etc. In this way, therewould be a rapid growth of the molecular chains. The synthetic routewith the progress to date is shown in Scheme II. ##STR6##

We have presently completed the synthesis of the tetramer 39.

Notice that the monomer 33 was converted to the activated systems 34 and35, and then coupled to form the dimer 36. Analogously, the dimer 36 wasactivated to 37 and 38, and then coupled to form the tetramer 39. Wehope to continue this approach to rapidly grow the molecular wires withdoubling length at each consecutive coupling. Note that each couplingstep utilized only a catalytic amount of palladium and copper andexcesses of base were used to satisfy the hydroxyl deprotonation as wellas the thiophene deprotonations. The next coupling will provide theoctamer, followed by the 16-mer, etc. However, if one wanted a chain of24-mer in length, coupling of a 16-mer with an octamer could analogouslybe accomplished. The hydroxyl functionalities provide a handle forsimple purification; however, if desired, they could easily be blocked(protected as the TBDMS ether) to prevent possible bonding to thelithographic surface. Hence, we have rapidly constructed molecular wiresand we have demonstrated a method to extend molecular chain lengthrapidly and efficiently. Thus, we have demonstrated that, from thesynthetic standpoint, thiophene-based chains are the optimal chains touse for molecular wire synthesis. Moreover, Scheme II outlined a methodto rapidly build molecular chains so that successive monomer utilizationis not necessary.

There are two initial synthesis objectives: Functionalize "alligatorclip" termini for adhesion of a single molecular chain tonanolithographic probes. The beauty of the thiophene and ethynylenemethodology becomes immediately apparent. Thiophenes and alkynes can beeasily deprotonated with strong lithium bases such as LDA oralkyllithiums. Thus an organolithium can be obtained. Since lithium isat the top of the electropositivity scale, we can transmetalate toalmost all other metals, or react with almost any electrophile.³⁰ Forexample, a thiophene end group can be lithiated and converted to a (1)thiol for adhesion to Au surface, (2) diarylphosphine for adhesion to Pdsurfaces, (3) carboxylated for adhesion to oxide surfaces, (4)transmetalated and cross-coupled to bipyridyls for adhesion to Fe, etc.(see Scheme III below). Note that the same thing could be done for thealkyne terminated ends. Moreover, through nickel and palladium-catalyzedcross-couplings, these units could be introduced at the outset of thesynthetic sequence, or at the last step. The introduction of these unitsat the last step would certainly prove to be more advantageous sincesimple modification to an existing chain would permit the affixing of avariety of end functionalities. Moreover, the ends of the chains in thethiophene-ethynylene systems (i.e. structure 39 in Scheme II) allowselective differentiation of the two ends. In this way, we could have,for example, one thiol cap and one phosphine cap. Deactivation of thepalladium catalyst by the thiol would be avoided by protection of thethiol as the t-butylthioether followed by Hg(OAc)₂ removal at the finalstage.³¹

Scheme III shows the planned process to extend the molecular chains toat least 100 Å in length. ##STR7##

Several options are available for fabrication of sub-100 Å contact probespacing. For monotype contacts, STM lithography, etching, or diamond-AFMtip³² scribing of a ˜50 Å gap in a metal wire 100-200 wide is possible.Sub-100 Å polytype metal contacts, for a selective attachment polarity(i.e., Au and Pt) is possible by a combination of step-edge and angleevaporation techniques. Alternatively, chain extension of the conjugatedoligomers, similar to the process outlined in Schemes I and II will beused to continue the chain length extensions. We use a combination ofpalladium and nickel-catalyzed cross-coupling reactions andpalladium-copper-catalyzed Sonogashira couplings. These are the mostadvanced and highest yielding carbon-carbon couplings for the requiredsynthesis.³³ The extension to 100 Å is achievable from thenanofabrication standpoint, and is desirable for variable lengthconnectivity considerations.

Moldeling will speed identification of the candidate organics that arestable, solution synthetic, conjugatable with selective attachment endsand barriers, have acceptable conductivity and mechanical strength, andattach strongly to particular metal groups with good ohmic contact.These are not necessarily compatible or automatic requirements. Itshould be stressed that complex device structure, s cannot be realizedwith "Edisonian" approaches, and that interactive modeling is key torealizing the goals of this program. Crucial to the success ofspontaneously assembled molecular circuits is a detailed understandingof the electronic and mechanical (thermoelastic) properties of thepolymers and junctions used as the fundamental assembly blocks of thedesign. A modeling effort to attack this problem decomposes into twophases; 1) construction of molecular wires and gluing pads, and 2)design of active gain elements. Modeling tools for detailing the orbitalstructure responsible for delocalization include:

Hartree-Fock methods, which give the most accurate results, but arelimited to approximately 70 atoms (on a Cray Y class machine or aConnection Machine -2).

MNDO (modified neglect of differential overlap) techniques, which caneasily deal with the wavefunctions or orbital structure of approximately250 atoms on sophisticated scientific workstations.

Extended Huckel methods, which can calculate orbital structure of

ten thousand atoms, but are sufficiently semi-empirical that they areconsidered unreliable for complex delocalization calculations.

The strategy will be to use MNDO methods to quickly identify candidates,cross-checking with detailed Hartree-Fock simulations to insure that theMNDO methods have given accurate results. The first objective is to findand analyze suitable conducting oligomers (monomers), examiningdelocalization as a function of unit size and the binding energy ofthese oligomers to each other. This will build a catalog of conductivechains which the experimentalists can interactively design with, andhave appropriate conductivity and stability. The second objective is toexamine the end group problem and the interaction of end groups withvarious metals, for selectivity and binding strength, satisfyingconductivity and stability constraints (the functionalization ofendgroups onto such short chains will alter the orbital behavior of thechain-endgroup complex, as well as when the endgroup is attached to themetal). An attractive flexibility of the fabrication approach is thatthe functionalized end group need not be restricted to just metalattachment. Attachment of selective molecules to various semiconductors(and oxides) has been demonstrated. For example, one could fabricate analignment substrate that is the combination of heterostructures andmetallic contacts (perhaps with appropriate vias in an overlayer) forvery general structures. This capability, as well as the ability todeposit the selective attachment oligomers from solution, and possibleCVD deposition of some critical steps (as in the to be described organicquantum dot work), gives the fabricator tremendous flexibility.

Further Modifications and Variations

It will be recognized by those skilled in the art that the innovativeconcepts disclosed in the present application can be applied in a widevariety of contexts. Moreover, the preferred implementation can bemodified in a tremendous variety of ways. Accordingly, it should beunderstood that the modifications and variations suggested below andabove are merely illustrative. These examples may help to show some ofthe scope of the inventive concepts, but these examples do not nearlyexhaust the full scope of variations in the disclosed novel concepts.

Note that self-aligned deposition from solution of contacts fromoligomer ends can be targeted to selectively to semiconductor contactareas, as well as to metal pads.

The conductive oligomers do not have to thiophene or thiophenederivatives. Alternative conductive oligomers can be used if theyprovide adequate conductivity and stability. In particular, it iscontemplated that doncutive oligomers with a ladder structure may beadvantageous for some applications.

Note that the conductive oligomers, and the charge-transfer structures,do not necessarily have to be organic compounds, although the vastexperience base of organic chemistry simplifies the fabrication detailedabove.

Note that the ultimate local environment of the conductive polymers doesnot strictly have to be dry, as in the presently preferred embodiment.Alternatively, the molecular electronic material can be allowed toreside in a solvent environment. This is preferably a nonpolar solvent,but could alternatively be aqueous or another polar solvent.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given.

What is claimed is:
 1. A picoelectronic device comprising:first andsecond conductor chains, each comprising multiple monomor units havingmutually conjugated bonding; a first barrier region, connected to saidfirst conductor chain, said first barrier having a potential energy forelectrons which is less favorable than that of said first conductorchain; a second barrier region, connected to said second conductorchain, said second barrier having a potential energy for electrons whichis less favorable than that of said second conductor chain; a wellregion, connected to said first and second barrier regions, said wellregion having a potential energy for electrons which is more favorablethan that of said first and second barrier regions; a third barrierregion, connected to said well region, said third barrier having apotential energy for electrons which is less favorable than that of saidwell; a third conductor chain, comprising multiple monomor units havingmutually conjugated bonding, and operatively connected to said wellregion through said third barrier region; whereby changing potentialsapplied to said third chain can effect modulation of currents betweensaid first and second chains.
 2. The picoelectronic device of claim 1,further comprising an electrical contact directly connected to saidthird conductor to control the potential thereof.
 3. The picoelectronicdevice of claim 1, further comprising a second well connected to saidthird barrier, and a fourth barrier connected to said second well, saidthird chain being operatively connected to said first well only throughsaid third and fourth barriers.